This invention relates to a loop seal insulation assembly for the loop seal piping of a pressurized water nuclear reactor system.
In a pressurized water nuclear reactor, a core of fissionable material is cooled by a flow of primary coolant which flows past the core during operation of the reactor and picks up heat emitted by the fissionable material. The primary coolant is under considerable pressure during this process. This high pressure is maintained by means of a pressurizer, an upright pressure vessel equipped with apparatus designed to maintain the primary coolant at the desired level.
A loop seal piping system is typically located at each of three different azimuths near the top head of the pressurizer. This piping has a generally S-shaped configuration and connects the pressurizer at one end with a safety valve at its other end. By virtue of its configuration, the typically uninsulated piping acts as a thermal buffer between the pressurizer and the safety valve by collecting cooled condensate upstream of the safety valve. Such a buffer is necessary because the safety valve is not rated for continuous high temperature service. The safety valve is pressure-sensitive and activates, i.e. opens, when excess pressure builds up in the pressurizer. The safety valve operates periodically in actual service, and protects the nuclear steam supply system from over-pressurization.
Problems in the operation of this valve have occurred in the past. For example, at the Three Mile Island nuclear facility, it has been observed that after activation of a safety valve in a loop seal assembly, the valve did not reseat properly. In this connection, it is known that water condenses inside conventional loop seal piping and accumulates in the curved section of the piping. When the valve is actuated, this slug of water will travel downstream and through the safety valve, having a destructive effect on valve components as well as a "water hammer" effect on downstream piping.
In order to ensure that less-destructive steam rather than liquid water flows through the safety valve during valve activation, while maintaining the temperature near the valve relatively low during normal operation, a temperature in the range of about 250.degree. F. to 350.degree. F., and preferably in the range of about 310.degree. F. to 350.degree. F. is required at the loop seal pipe/safety valve interface.
Various means have been proposed to accomplish the maintenance of such temperature ranges within the loop seal piping. For example, electrical heat tracing could be used. Such a system would have the disadvantage of high cost, however. Additionally, such a system would be an "active" one requiring procedures and backup apparatus to insure its operability under all conditions. Of course, a massive redesign of downstream piping and safety valve components and restraints to withstand the damaging effects of water transport at high velocities is possible, but such modifications are time consuming and costly.
The use of conventional pipe insulation would appear to be a practical alternative. Insulation is presently used on the upper part of the pressurizer and sometimes on a portion of the loop seal piping nearest the pressurizer. However, careful analysis of this alternative has demonstrated that simply insulating the entire length of loop seal piping does not transfer enough heat to raise the temperature in the vicinity of the loop seal pipe/safety valve interface to the desired level.
It has been found that the use of reflective type insulation combined with exposure of part of the pressurizer surface to the loop seal piping, has provided the needed temperature range in the piping, and in particular at the loop seal/valve interface. The invention takes advantage of radiative and convective as well as conductive modes of heat transfer to achieve the desired temperature elevation by removing a portion of existing insulation typically present on the upper portion of a pressurizer. The area on the upper portion of the pressurizer from which insulation is removed is defined by analysis for any particular system, and in no case is greater than the area defined by the inner case of the reflective type insulation.
The practice of the present invention provides a relatively inexpensive means of substantially reducing damage to safety valve components and downstream piping. It is a passive system and therefore reliable, not depending on outside power sources or the like. Furthermore, reflective type insulation has proven to be a low-maintenance material in other applications.
A major advantage of this invention is the inherent flexibility of the system, which allows adjustment of individual insulation panels to produce desired temperature variations as required. Safety valve allowable sustained operating temperature requirements are considered in the design of the insulation system. Thus, the elevated temperature obtained by the practice of the invention should not exceed allowable sustained operating temperatures for the valve.